The present invention relates to a process for evaluating performance deterioration of a nitrogen oxide storage catalyst which has a nitrogen oxide storage function and an oxygen storage function and is operated with cyclic alternation of the standardized air/fuel ratios in the exhaust gas from air/fuel ratios of greater than 1 (sorption phase) to store the nitrogen oxides to standardized air/fuel ratios of less than 1 (desorption phase) to desorb and convert the nitrogen oxides.
Nitrogen oxide storage catalysts were developed specifically for the treatment of exhaust gases from lean operating internal combustion engines. Lean operating gasoline engines, so-called lean-burn engines and diesel engines belong to the group of lean operating internal combustion engines. Lean-burn engines, in particular with direct injection of gasoline, are being increasingly used in the vehicle construction sector since they enable theoretical fuel savings of up to 25% as compared with stoichiometrically operated internal combustion engines.
Nitrogen oxide storage catalysts have the ability to store nitrogen oxides over a wide temperature range under oxidizing exhaust gas conditions, that is to say under lean conditions. This operation mode is therefore also called the sorption phase in the following.
Since the storage capacity of a storage catalyst is limited, it has to be regenerated from time to time. For this purpose, the standardized air/fuel ratio of the air/fuel mixture supplied to the engine and thus also the standardised air/fuel ratio of the exhaust gas leaving the engine is lowered to values below 1 for short periods. This is also called enrichment of the air/fuel mixture or the exhaust gas. Therefore reducing conditions prevail in the exhaust gas upstream of the entrance to the storage catalyst during this short enrichment phase.
Under the reducing conditions during the enrichment phase, the stored nitrogen oxides are released and reduced to nitrogen on the storage catalyst, with simultaneous oxidation of carbon monoxide, hydrocarbons and hydrogen in the same way as in conventional three-way catalysts. This operation mode of the storage catalyst is also called the desorption and conversion phase in the following. With correct functioning of the overall system of storage catalyst, oxygen sensors and engine electronics system, approximately stoichiometric conditions prevail downstream of the storage catalyst during the desorption phase, that is to say the hydrocarbons and carbon monoxide present in excess upstream of the storage catalyst during the desorption phase are oxidized on the storage catalyst by the released nitrogen oxides.
The duration of the sorption phase typically lasts about 30 to 100 seconds. The duration of the desorption phase is substantially shorter and is within the range of less than only a few seconds (1 to 10 seconds).
The mode of operation and composition of nitrogen oxide storage catalysts are disclosed, for example, in EP 0 560 991 B1. These catalysts contain at least one component from the group of alkali metals (potassium, sodium, lithium, cesium), alkaline earth metals (barium, calcium) or rare earth metals (lanthanum, yttrium) as storage components. The storage catalyst contains platinum as a catalytically active element. Under oxidizing exhaust gas conditions, that is under lean operation, the storage components store the nitrogen oxides contained in the exhaust gas in the form of nitrates. For this to occur, however, the nitrogen oxides, about 50 to 90% of which are present as nitrogen monoxide, depending on the construction of the engine and its mode of operation, first have to be oxidised to nitrogen dioxide. This takes place on the platinum component in the storage catalyst.
In addition to the components mentioned above, the nitrogen oxide storage catalyst also contains oxygen storing components. In this case it can take on the functions of a conventional three-way converter catalyst in addition to storing nitrogen oxides. Cerium oxide is used for the most part as an oxygen storing component. The nitrogen oxide storage catalyst then also has an oxygen storage function in addition to its nitrogen oxide storage function and thus it is bifunctional.
An important problem with modern exhaust gas treatment process is evaluating performance deterioration of the catalysts used in order to facilitate exchanging catalysts which are not working properly anymore. This also applies to nitrogen oxide storage catalysts, the nitrogen oxide storage capacity of which can be damaged on the one hand by sulfur present in the fuel and on the other hand by thermal stresses. Whereas poisoning due to sulfur can generally be reversed at elevated temperatures, thermal damage is an irreversible process.
With bifunctional storage catalysts, in principle both storage functions can be damaged by poisoning and by thermal effects. Damage to one function, however, does not necessarily imply damage to the other function.
Since nitrogen oxides and oxygen are both oxidizing components, their effects cannot be clearly separated from each other, so wrong diagnoses may be made when checking the catalyst. Therefore, there is a fundamental need to be able to assess the proper functioning of the two storage functions independently of each other.
EP 0 690 213 A1 describes an exhaust gas treatment device which is able to determine the degree of damage to a nitrogen oxide storage catalyst or to a three-way converter catalyst. For this, an oxygen sensor, the output signal from which is proportional to the air/fuel ratio of the exhaust gas, is arranged downstream of the nitrogen oxide storage catalyst or three-way converter catalyst. To determine the damage to a nitrogen oxide storage catalyst or three-way converter catalyst, from time to time the air/fuel ratio in the fuel mixture is altered from lean to rich or from rich to lean. During the operating period with an altered air/fuel ratio, the damage already caused to the catalyst is determined from the peak value of the output signal from the oxygen sensor.
A process for checking a bifunctional nitrogen oxide storage catalyst is not disclosed in the document mentioned above. Therefore, an object of the present invention is to provide a process for evaluating the performance deterioration of bifunctional storage catalysts which enables the two storage functions to be checked separately.